1. Field of the Invention
The present invention relates to an overheat protection circuit for power supply devices and a direct-current power supply device, and in particular relates to an overheat protection circuit for a switching power-supply device (DC-DC converter), and so on.
Priority is claimed on Japanese Patent Application No.2005-366293, filed Dec. 20, 2005, the content of which is incorporated herein by reference.
2. Description of the Related Art
In power supply devices (for example, a switching power-supply device), smoke and ignition which are connected to fire are critical issues that must never arise. As a means for preventing these issues, an overheat protection circuit (overload protection circuit) is often provided.
In most conventional power supply devices, dedicated thermosensors such as thermostats, thermistors and posistors are used to prevent overheating. These components are used in a small number and require elaborate control for temperature so as to be costly, which is a disadvantage. Therefore, in a conventional power supply device disclosed in PCT International Patent Application, Publication No. WO 2004/017507 filed by the present assignee, an inexpensive overheat protection method was proposed in which a Schottky barrier diode (SBD) was used. Hereinafter, a description will be made for the conventional power supply device.
FIG. 4 shows an example of the overheat protection circuit used in the conventional power supply device. In the direct-current power supply device 2 shown in FIG. 4, an alternating-current power supply AC is subjected to rectification and smoothing by using a diode bridge DB1 and a capacitor C1 to obtain a direct-current power supply. The direct-current power supply thus obtained is switched by using a switching element Q1 and applied to a primary winding P of a transformer T.
The transformer T stores magnetic energy when the switching element Q1 is kept on, and releases the stored magnetic energy from a secondary winding S of the transformer T when being kept off. This energy is rectified and smoothed by a rectifier diode D51A and a capacitor C51 and supplied to a load as direct voltage.
The switching element Q1 is subjected to on/off control by a gate control terminal G of a control circuit CONT. A resistor R1 is a starting resistor for starting the control circuit CONT, and charges a capacitor C2 for control power supply to start the control circuit CONT on loading of the AC power supply. When the control circuit CONT is activated, the switching element Q1 starts on and off operations. Thereby, voltage develops at a control winding C of the transformer T, which is rectified and smoothed by a diode D4 and the capacitor C2 to maintain the control power supply of the supply control circuit CONT.
The control circuit CONT is provided with a feedback terminal (FB). Output voltage of a capacitor C51 is detected by an output-voltage detecting circuit 10 including resistors R52, R53, R54, a shunt regulator Z51, a capacitor C52 and a photocoupler PC1 (the photocoupler constituted with a light emitting diode PC1-D and a light receiving transistor PC1-TR), and the error signal is input via the photocoupler PC1 into FB terminals of the control circuit CONT. This error signal provides the switching element Q1 with PWM (pulse width modulation) control, thereby the output voltage is kept constant.
The control circuit CONT is also provided with an overvoltage latch circuit. When the output voltage of the capacitor C51 is in an overvoltage state, the overvoltage is detected by an overvoltage detecting circuit 11 including a zener diode D52, a resistor R51, a photocoupler PC2 (the photocoupler constituted with a light emitting diode PC2-D and a light receiving transistor PC2-TR), and the detected signal is input via the PC2 into an overvoltage detecting terminal (OVP) of the control circuit CONT.
A high-level voltage is applied to the overvoltage detecting terminal (OVP) to set a flip flop inside the control circuit CONT, thereby a drive signal of the switching element Q1 output from a gate control terminal G of the control circuit CONT is blocked. Thereby, the overvoltage latch circuit is activated to block the drive signal of the switching element Q1 making it possible to allow the safe shutdown of a power supply device. Furthermore, the control circuit CONT is able to keep the shutdown by an extremely small flow of electricity supplied from the starting resistor R1.
This type of overheat protection used in the conventional power supply devices is to use reverse leakage current Ir of the SBD D51, activating an overvoltage detecting circuit 11 to shut down a power supply device. For example, the SBD D51B for overheat protection thermally coupled with an output rectifier diode D51A is connected in parallel with respect to an overvoltage detecting zener diode D52.
Thereby, a direct-current power supply device 2 is kept in an overheat state (overload state). When the rectifier diode D51A is heated, the SBD D51B thermally coupled with the D51A is heated up to a similar temperature, thus an increase in reverse leakage current Ir occurs. The reverse leakage current Ir of the D51B flows to the light emitting diode PC2-D of the photocoupler PC2. Thereby, a signal similar to a state of detecting overvoltage into an OVP terminal of the control circuit CONT via a light receiving transistor PC2-TR of the PC2 is inputted to shut down the direct-current power supply device 2.
In the Schottky barrier diode (SBD) D51B, the reverse leakage current Ir at high temperatures is greater than generally-used PN junction diodes. Leakage current is abruptly increased at the junction temperature of approximately 120° C. The Schottky barrier diode (SBD) D51B detects leakage current, thereby making it possible to reliably provide overheat protection before the conventionally-used PN junction-type semiconductors are put into a dangerous state. This circuit can be constituted by adding only one SBD to individual outputs of a power supply device having a plurality of outputs in particular. Therefore, the overheat protection circuit can be installed in each output at a very inexpensive price. Furthermore, the output rectifier diode D51A and the SBD D51B are thermally coupled, and advantageously, they can be formed in an integral manner to make the thermal coupling more dense. These can be easily constituted by using a TO-220 package or a TO-3P package.
As described above, in the overheat protection circuit for the direct-current power supply devices shown in FIG. 4, the reverse leakage current Ir of the SBD D51B is used to activate the overvoltage detecting circuit 11, thereby the direct-current power supply device 2 is shut down.
In the overheat protection circuit shown in FIG. 4, the overvoltage detecting circuit 11 mounted on the secondary side of the transformer T is used to provide overheat protection. In an attempt to constitute a power supply device more inexpensively, overvoltage is often detected on the primary side of a transformer T. Therefore, it has been desired to constitute an overheat protection circuit with a SBD at an inexpensive price, even in a case where no overvoltage detecting circuit 11 is mounted on the secondary side (output side) of the transformer T.